COMMENTS The Westermark is an eponym indicating the abrupt cutoff of pulmonary vascularity distal to a large central pulmonary embolus. The presumed mechanism behind the image arises from the nearly complete obstruction of bloodflow to the pulmonary artery distal to the embolic clot.Presumably the lack of flow to these more distal vessels results in their radiographic transparency and an appearance of an abrupt truncation as is shown in this exemplary case.